This is tracked by defect QM002320808 and only affects BPPM 9.x.
The problem is in page C:\Program Files\BMC Software\ProactiveNet\pw\tomcat\webapps\pronto\jsp\PasswordChange.jsp.
While the new character limit is expressed as a variable (%=passwordMaxLength%), the old password limit is hardcoded to 15. Offending line:
<td width=10 height=2><input type=password name=oldpasswd maxlength=15 autocomplete="off"></td>
If you need to change the password for a user which has a password greater than 15 characters, the preferred method is to use the command line:
pw user change_passwd <user_name> -old <old password> -new <new password>
However, to use the BPPM Operations Console to make the change, you can edit the PasswordChange.jsp file and change maxlength=15 to maxlength="<%=passwordMaxLength%>". Restart the BPPM server for the change to take effect.